1. Field of the Invention
The present invention relates generally to a liquid crystal display unit, and more specifically to an in-plane switching mode active matrix liquid crystal display unit.
2. Description of Related Art
It is known in the art that there is an in-plane switching (IPS) mode liquid crystal display (LCD) unit in which the axes of the orientated liquid crystal molecule are rendered rotatable in a plane substantially parallel with the substrates forming part of the display unit. With such an in-plane switching mode, a TFT (Thin Film Transistor) substrate, which is arranged in parallel with a color filter substrate, carries thereon common electrodes and pixel electrodes. Image display is implemented using the electronic field components substantially in parallel with the substrates, which electric field components are generated by applying electric potential to the common and pixel electrodes. The in-plane switching mode liquid crystal display unit has round advantages that a wide view angle is obtained and the contrast of a displayed image is improved.
A liquid crystal display unit is as is well known in the art, provided with a first substrate (viz., TFT substrate) and a second substrate (viz., color filter substrate). These two substrates are positioned in parallel with each other in an opposing manner and hold a liquid crystal layer therebetween. The first substrate carries thereon scan lines, common lines in parallel with the scan lines, data lines provided such as to extend crosswise to the scan lines, and active electronic elements (viz., TFTs) provided nearby the intersections of the data lines and the scan lines. The second substrate is provided with a color filter, a deflector plate, etc.
With the in-plane switching mode, in order to rotate the axes of orientated liquid crystal molecules in a plane substantially parallel with the substrates, two kinds of transparent comb-teeth electrodes are patterned on the first substrate such that the comb-teeth electrodes of the two different comb electrodes are alternatively positioned and connected respectively to the common electrodes and the pixel electrodes. A liquid crystal display unit using the in-plane switching mode is disclosed in, by way of example, Japanese Laid-open Patent Application No. 2000-352713 (first prior art).
The conventional in-plane switching mode display unit has encountered the difficulty that it is not easy to sufficiently rotate the liquid crystals above the comb-teeth electrodes because the lateral electric fields thereabove are poor. Furthermore, the liquid crystal molecules are liable to rise up above the comb-teeth electrodes due to the electric fields normal to the substrate, resulting in the fact that an effective retardation is reduced. Accordingly, even if the comb-teeth electrodes are made of a transparent material, the light transmissivity (viz., light transmission ratio) above the comb-teeth electrodes does not reach the transmissivity at the area between the comb-teeth electrodes, and thus it is difficult to achieve a high level of panel transmissivity.
Second prior art (viz., Japanese Laid-open Patent Application No. 2003-75850) discloses an IPS mode active matrix LCD unit, which is provided with uneven or rugged portions in a pixel's light transmitting area in order to vary liquid crystal gaps thereby to improve liquid crystal response. However, the comb-teeth electrodes of the second prior art are not transparent and thus the liquid crystals above the comb-teeth electrodes provides no contribution to the increment of light transmissivity. Further, even if the comb-teeth electrodes are made of transparent material, since there exists no special intension of increasing the liquid crystal gaps above the comb-teeth electrodes, it is not possible to increase the light transmissivity above the comb-teeth electrodes.
Third prior art (viz., Japanese Laid-open Patent Application No. 2003-215620) is concerned with a technology of forming a concaved portion(s) between the teeth electrodes so as to provide different crystal gaps within each pixel area. This prior art increases the liquid crystal gaps between the comb teeth electrodes in order to make it easy to rotate liquid crystals between the comb-teeth electrodes with the result of improving response time of halftone display images. Therefore, there is no need to increase the number of comb-teeth electrodes merely for the purpose of response time improvement, and accordingly, it is possible to prevent undesirable aperture ratio decrease resulting from the increase in the number of comb-teeth electrodes. In other words, the third prior art is able to increase aperture ratio by reducing the number of comb-teeth electrodes without adversely affecting the response speed. However, the third prior art has encountered a difficulty that it is not possible to improve the light transmissivity without decreasing the number of comb-teeth electrodes. In the case where the number of comb-teeth electrodes is small, it is not practically acceptable to further reduce the number of comb-teeth electrodes. This is because the reduction of the number of comb-teeth electrodes widens the interval between the adjacent, comb-teeth electrodes, leading to the difficulty that the liquid crystal drive potential is undesirably increased.
Fourth prior art (viz., Japanese Laid-open Patent Application No. 2008-225337) discloses an active matrix liquid crystal display unit wherein an upper substrate is provided with a plurality of slanted surfaces on the liquid crystal layer side. The fourth prior art describes that a high aperture ratio can be obtained with the provision of the aforesaid slanted surfaces. However, the fourth prior art fails to provide any suggestion of improving the rotation of the liquid crystal molecules above the transparent electrodes provided in each pixel area.